In the days of double-digit inflation it was in a sense easier to make larger pay claims, simply in terms of getting a “cost of living” wage rise. Today the official inflation figures are steady at around (or under) 2% - even though life does not get any cheaper. But be prepared to be told that anything over 2% is “inflationary” or “unaffordable”.
The question is: how much are you undervalued already? What do people doing similar jobs get from their employer? Have you hit or surpassed targets, and played a vital role in your firm's success? If so, you are due more. Don't be embarrassed about asking – “shy bairns get nowt”, as they say. If you do have a trade union representative (see 'haggling and negotiating' next) get them to help you build a case. If you have to go in alone, aim high. If your boss is sensible, and knows how valuable you are, he/she will have to try and get near your figure. (They will have to defend their decision to their boss!) Your manager doesn't know if you have another job offer in your pocket (but don't pretend you do if you don't!) He/she doesn't know if you would leave – and as a talented person you would find work elsewhere.